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Sound is caused by vibrations which transmit through a medium and reach the ear or some other form of detecting device. Sound is measured in loudness (decibels (dB)) and frequency (Hertz (Hz)).
Airborne sound (or airborne noise) is sound that is transmitted through the air.
Typically, airborne sound might be generated by:
- Television and radio.
- Animal sounds such as dogs barking.
This is as opposed to structure-borne sound that results from an impact on or a continuous vibration against a part of a building fabric resulting in sound being radiated from an adjacent vibrating surface. An example of structure-borne sound is footsteps of a floor being heard in a room below.
Whilst they are sometimes considered to be separate phenomena, airborne and structure-borne sound are related, in that airborne sound can cause structure-borne sound and vice versa. Airborne sound may cause an element of the building fabric to vibrate when it comes into contact with a surface, and structural vibrations may radiate from a surface, generating airborne sound.
Poor detailing or poor standards of workmanship can result in airborne sound transmitting directly between spaces, for example through gaps around the edge of doors, and may result in flanking sound, where sound travels around a separating element, even though the element itself might provide very good acoustic insulation. Even very small gaps can cause a significant increase in the transmission of airborne sound.
Problems can also occur where doors, windows or other openings face onto ‘noisy’ spaces, such as a circulation space, a busy road or a school playground. If this deters occupants from leaving elements of the building open, this can affect the performance of natural ventilation strategies.
The amount of airborne sound in a space can be reduced by acoustic absorption, which reduces the amount of sound reflecting back into the space from the surfaces enclosing it, by acoustic insulation which reduces the amount or sound transmitting into the space from an adjacent space through the building fabric and by the elimination of gaps that might permit direct transmission.
Airborne sound transmission can be tested by placing a loudspeaker in a space to generate sound at a range of frequencies, and detecting any resulting sound in an adjacent space with a microphone. The difference is then calculated and adjustment made to take into account the sound absorption characteristics of the ‘receiving’ space. Tests are typically carried out in the range from 125 Hz to 4000 Hz.
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